Air bags of occupant protection systems are expensive and in certain circumstances are dangerous. It is therefore desirable to prevent deployment when the seat is empty to save the cost of replacement. It is important to prevent deployment when the seat is occupied by a child or by a very small adult if the occupant might be injured by deployment.
Occupant protection systems typically include a "sensor and diagnostic module" or "SDM" which performs various functions related to sensing the occurrence of a vehicle crash, monitoring various elements of the occupant protection system for proper operation and initiating deployment of occupant protection devices. SDM's typically include a microprocessor, an accelerometer responsive to decelerations parallel with the vehicle axis, an arming sensor, circuitry combining the aforementioned components to function together and switches for initiating deployment of the occupant protection devices.
Many systems are known that sense properties of the occupant of a seat. Certain of these systems contain elements for weighing the occupant. If the weight is very small it may be assumed that the seat is unoccupied or occupied by a small child, and in either case deployment would not be desired. If the weight is intermediate, say between 25 and 40 kilograms, then the occupant is likely to be a child and whether or not an airbag should be deployed depends on factors such as how energetically the airbag deploys. If the weight is greater than 45 kilograms the seat occupant is likely to be an adult who would be protected by an airbag.
Three types of weight sensing systems for installation in vehicle seats are known: A first type of weight sensing system comprises an array of force sensors located immediately beneath the upholstery material of the seat cushion which operates to measure the pressure of the occupant against the seat at the points where sensors are located. These sensors are typically responsive to small forces applied over a small area and an array of force sensors tells a microprocessor the magnitude and distribution of the force the occupant applies to the cushion. The microprocessor ascertains the weight and other characteristics of the seat occupant from the information provided by the array of force sensors.
A second type of weight sensing system is useful in the type of seat having a fabric covered seat cushion of foam rubber-like material supported by a platform. The second type of weight sensing system includes, typically, four force sensors located at the four corners of the platform between the platform and the frame of the seat. The outputs of the four sensors are added to ascertain the total weight being supported by the platform and, therefore, by the seat cushion.
A third type of weight sensing system comprises sensors for sensing stress in structural members of the seat. For example, a chair with a force sensor sensing the weight carried by each leg. The outputs of the four (in the case of the example) sensors are added to ascertain the total weight of the occupant.
All known embodiments of the aforementioned weight sensing systems fail to perform as desired in certain situations and no design is widely accepted at this time. In vehicles where the seat belts are attached to the structure of the vehicle all known weight sensing systems present the concern that a tightly belted child seat may impose stresses that make it appear to be an adult to the weight sensing system. All known designs must take into account that the feet of most normally seated adults will be resting on the floor so that some of the weight will not be seen by a weight sensing system, whereas a child or very small adult will apply all weight to the seat.
Force sensors placed immediately beneath the seat upholstery complicate manufacture and may affect the feel of the seat as sensed by the occupant.
It is well known to sense a force by applying it to a piston to generate hydraulic pressure sensed by a pressure sensor calibrated in units of force.
Seat occupant weight sensing systems responsive to stress in the seat structure must respond only to forces related to the weight of the seat occupant and not to stresses resulting from thermal expansion or attachment to the vehicle. This is not always easily achieved. However, seat structure force sensors are known to advantageously solve the aforementioned problem of belt forces causing a child seat to appear to be an adult: By anchoring the seat belts to the seat and placing the force sensors below the belt anchors the belt forces are not included in the weight.
The type of sensor wherein the weight of an occupant sitting on a cushion is transmitted though the cushion and sensed at the platform supporting the cushion may fail to register an accurate weight because a fraction of the occupant's weight may be supported by the back of the seat rather than on the seat cushion and, therefore, not be sensed. Also, the fraction of the occupant's weight supported by the seat back varies with the angle to which the back is reclined.
Seat backs that can recline typically expose a larger area of cushion at greater seat back recline angles. This causes the weight of the head and body of an occupant to be applied farther toward the rear of the seat cushion when the seat back is reclined.
Semiconductor pressure sensors are manufactured in large quantities by micromachining silicon wafers. Designs are based on various technologies and physical principles. Many of these sensors require additional circuitry to achieve a useful function. Typically, an integrated circuit complements the micromachined pressure sensing element. Certain of these sensors are suitable for operation submersed in liquid and operate by sensing the pressure in the liquid.
Accelerometers responsive to accelerations between plus and minus one or two times the acceleration of gravity are readily available from several suppliers. One supplier is Entran Devices, Inc. of Fairfield, N.J.
Seats in commercial production include sensors indicating the position of the seat on its track and the amount that the back of the seat is reclined.
Injection blow molding is a highly developed technology for making beverage containers. By this process a hollow piece of injection molded plastic is heated and placed in a form after which pressurized gas is blown into the hollow of the molded plastic. The pressure expands the heated plastic until its expansion is limited by a form. After cooling the completed product is removed from the form.
It is well known to connect a sensor using only two electrical conductors. The conductors simultaneously supply power and carry pulses of current over and above the current required to power the sensor. The width or magnitude of the pulses of current are modulated to indicate the physical quantity being sensed.
By the rules of Physics torque is always specified with respect to an axis. A torque about an axis may be caused by a force (considered to be a vector quantity) applied to a point that is not on the axis. The torque (also a vector quantity) is defined to be the vector product of the force vector times a distance vector from the point where the force is applied along a perpendicular to the axis. Hereinafter, only the scaler magnitude of the torque vector is discussed and it is called "torque". Hereinafter, "torque" resulting from a force applied at a point is defined to be the product of the magnitude of the component of the force vector in the direction perpendicular to both the axis and a perpendicular line from the point where the force is applied to the axis times the distance along the perpendicular from the point where the force is applied to the axis.
A general object of this invention is to provide a seat occupant sensing system offering low cost and superior performance and also to provide a force sensor that is particularly adapted for sensing force derived from the weight of a seat occupant which also overcomes certain disadvantages of the prior art.